For crossing distances greater than several meters, RS-232 is supplemented by the RS-422
standard. RS-422 can provide communications across more than 1.2 km at moderate bit rates such as 9.6 kbps. It is a differential , or balanced , transmission scheme whereby each logical signal is represented by two wires rather than one. RS-232 signals are single-ended
, or unbalanced , signals that drive a particular voltage onto a single wire. This voltage is sensed at the receiver by measuring the signal voltage relative to the ground potential of the interface. Over long distances or at very high speeds, single-ended transmission lines are more subject to degradation resulting from ambient electrical noise. A partial explanation of this characteristic is that the electrical noise affects the active signal wire unequally with respect to ground. Differential signals, as in RS-422, drive opposing, or mirrored, voltages onto two wires simultaneously (RS-422 is specified from ±2 to ±6 V). The receiver then compares the voltages of the two wires together rather than to ground. Ambient noise tends to affect the two wires equally, because they are normally twisted together to follow the same path. Therefore, if noise causes a 1-V spike on one-half of the differential pair, it causes the same spike on the other half. When the two voltages are electrically subtracted at the receiver, the 1-V of commonmode
noise cancels out, and the original differential voltage remains intact (subject, of course to natural
attenuation over distance). The difference between RS-232 and RS-422 transmission is illustrated Because of the longer distances involved in RS-422 interfaces, it is not common to employ the standard set of hardware handshaking signals that are common with RS-232. Therefore, some form of software handshaking must be implemented by the end devices to properly communicate. Some applications may not require any flow control, and some may use the XON/XOFF method. RS-422 does not specify a standard connector. It is not uncommon to see an RS-422 transmission line’s bare wire ends connected to screw terminals. Another common difference between RS-422 and RS-232 is transmission line termination . Transmission
line theory can get rather complicated and is outside the scope of this immediate discussion.
standard. RS-422 can provide communications across more than 1.2 km at moderate bit rates such as 9.6 kbps. It is a differential , or balanced , transmission scheme whereby each logical signal is represented by two wires rather than one. RS-232 signals are single-ended
, or unbalanced , signals that drive a particular voltage onto a single wire. This voltage is sensed at the receiver by measuring the signal voltage relative to the ground potential of the interface. Over long distances or at very high speeds, single-ended transmission lines are more subject to degradation resulting from ambient electrical noise. A partial explanation of this characteristic is that the electrical noise affects the active signal wire unequally with respect to ground. Differential signals, as in RS-422, drive opposing, or mirrored, voltages onto two wires simultaneously (RS-422 is specified from ±2 to ±6 V). The receiver then compares the voltages of the two wires together rather than to ground. Ambient noise tends to affect the two wires equally, because they are normally twisted together to follow the same path. Therefore, if noise causes a 1-V spike on one-half of the differential pair, it causes the same spike on the other half. When the two voltages are electrically subtracted at the receiver, the 1-V of commonmode
noise cancels out, and the original differential voltage remains intact (subject, of course to natural
attenuation over distance). The difference between RS-232 and RS-422 transmission is illustrated Because of the longer distances involved in RS-422 interfaces, it is not common to employ the standard set of hardware handshaking signals that are common with RS-232. Therefore, some form of software handshaking must be implemented by the end devices to properly communicate. Some applications may not require any flow control, and some may use the XON/XOFF method. RS-422 does not specify a standard connector. It is not uncommon to see an RS-422 transmission line’s bare wire ends connected to screw terminals. Another common difference between RS-422 and RS-232 is transmission line termination . Transmission
line theory can get rather complicated and is outside the scope of this immediate discussion.
